In automotive ventilation systems it is customary and desirable to afford both heating and cooling functions whether the cooling is accomplished by air conditioning or by ambient air. The systems operate in three modes: heating, air conditioning and defrost or defog. Each mode directs the air into the vehicle differently from the other modes. For heating mode, the air is directed to the vehicle floor; for air conditioning mode, the air is directed through instrument panel vents; and for defog/defrost mode, the air is directed onto the windshield. In any mode, a variable blend of hot and cool air is possible for temperature control and for heating and air conditioning modes unblended hot and cool air, respectively, can be selected.
In such systems it is known to provide a mixing chamber having inlet and outlet doors and a valve arrangement for controlling the proportion of hot and cold air streams entering the chamber for temperature selection and another valve arrangement for controlling the mode of air distribution from the outlet. Separate controls operate the two valve arrangements. Generally, flapper valves are used for both temperature and mode functions to selectively open or close various openings, although in some cases a rotary valve such as an apertured drum is used. Typically, a separate actuator is used for each valve or a complex linkage or cam arrangement is used to control plural valves. In any event, separate actuators are used for temperature control and mode control. Thus the use of several independently operated valves requires several actuators. Further, the valves each take up a certain amount of space and thus hinder attempts to design compact units.
For air conditioning applications it is commonplace to cool the air entering the mixing chamber by an evaporator positioned in the air stream just upstream of the mixing chamber inlets to condition all the incoming air. A heater core, inside or outside the chamber, heats only that portion of the incoming air which passes through it. The temperature control valves determine whether all, some or none of the air is heated.